The ErbB-2 oncoprotein contributes to the ability of various adenocarcinomas to metastasize and resist chemotherapy, but the underlying molecular mechanisms are still unknown. Because ErbB-2 belongs to a family of tyrosine kinase receptors, that includes the EGF recpetor (ErbB-1) and two receptors for NDF/heregulin (ErbB-3 and ErbB-4), it is thought that ErbB-2 is activated by a ligand that has so far eluded detection (vertical signaling model). However recent evidence raised the possibility that ErbB-2 functions in tumor cells solely as a ligand-less pan-ErbB signaling subunit of EGF and NDF-receptors (lateral signaling model). Here we propose to contrast these two models by employing a combination of genetic , biochemical and immunological approaches: (1) Genetic- transgenic mice expressing a soluble ErbB-2 fusion protein and animals expressing ErbB-2 mutants that are selectively defective in vertical signaling will presumably reflect the phenotype of the ErbB-2 ligand. (2) Vertical signaling - a conventional soluble ligand will be searched by using a low-background chimera between ErbB-2 (extracellular) and ErbB-1 (intracellular). Membrane-bound ligands will be searched by using soluble versions of ErbB-2 , whereas an antagonist ligand, similar to the Drosophila protein Argos, will be secreened by expression cloning. Phage display peptide libraries will be screened to identify a synthetic analog of the elusive ligand. (3) Lateral signaling - The hierarchy and structural basis of ErbB-2 crosstalk will be identified. Signaling downstream of each heterodimeric receptor combination will be investigated, and an attempt will be made to identify additional ErbB proteins by using expression cloning. (4) signaling inhibition - mAbs to ErbB-2 and its partners, intracellular antibodies, soluble receptors, natural or syntheitic ErbB-2 ligands and synthetic blockers of receptor dimerization will be examined for inhibition of signal transduction and potential therapeutic utility. The proposed research is expected to yield a comprehensive mechanistic understanding of the tumorigenic action of ErbB proteins and also identify new molecules, such as natural or synthetic ligands, soluble receptors and specific antibodies, that may have prognostic and therapeutic values.